How redshift colours our view of the history of the universe

Redshift makes objects at different distances appear in different hues – allowing us to reconstruct how dark matter and energy have shaped cosmic evolution

By Andrew Pontzen

HOW can we make an accurate map of the universe when telescopes deliver only a two-dimensional picture? Measuring how far away cosmic objects are is no trivial affair – we can’t move our surveying equipment or lay a tape measure between galaxies.

For the best part of a century, the answer has been to measure redshift. In the 1920s, astronomer Edwin Hubble and others discovered that light from galaxies beyond the Milky Way is consistently shifted to longer, redder wavelengths.

Scroll down for an interactive graphic that shows how redshift can help explain the evolution of the universe

This was the first inkling that the universe began in a big bang, and has been expanding ever since. The reason for the shift in colour is that as space-time expands, it stretches the light passing through it.

The more expanding space the light has passed through, the greater the degree of this redshift, so far-off objects appear redder. We also see these objects as they were earlier in the universe’s history because of the length of time the light has been travelling.

Redshift measurements have brought surprising discoveries, not least that the universe’s expansion has apparently begun to accelerate, something attributed to an enigmatic “dark energy”. Cosmic maps extending to even more distant objects and covering more of the sky should help us work out what is going on – provided our redshift measurements are accurate (see “Why we can’t work out where everything is in the universe”).

Shift key

The more an object’s light has shifted to longer, redder wavelengths, the greater the distance through expanding space the light has travelled, and the further back in time ...

To continue reading this premium article, subscribe for unlimited access.